The present invention relates to an analytical method for determining formaldehyde in an electroless copper plating bath.
An electroless copper plating bath usually contains cupric ions, a complexing agent that forms a complex compound with cupric ions, and formaldehyde as a reducing agent. It has a comparatively high pH value. As the electroless copper plating proceeds, the concentrations of cupric ions and formaldehyde decrease and the pH goes down. The change in concentrations lowers the deposition rate of electroless copper plating and fluctuates the physical properties of the electroless copper deposit. Thus it is necessary to measure the concentrations of cupric ions and formaldehyde in the electroless copper plating bath and the pH of the bath continuously or intermittently, and to keep a prescribed level of concentrations by replenishing them and to keep the pH at a prescribed value.
Heretofore, there have been proposed various methods for analyzing the components of the electroless copper plating bath. For example, the quantity of formaldehyde in the plating bath is determined by using a sulfite (sodium sulfite). This method utilizes the addition reaction of formaldehyde with sodium sulfite. According to this method, the plating solution is neutralized to a certain level of pH with a prescribed amount of acid aqueous solution of predetermined concentration, and then sodium sulfite is added to the neutralized plating solution. Formaldehyde reacts with sodium sulfite to give sodium hydroxide as shown in the following equation. EQU HCHO+Na.sub.2 SO.sub.3 +H.sub.2 O.fwdarw.CH.sub.2 (NaSO.sub.3)OH+NaOH
The sodium hydroxide thus formed changes the pH. Therefore, the quantity of formaldehyde can be determined by measuring the pH change with a pH meter, or by determining the quantity of sodium hydroxide with acidimetry to restore the original pH of the neutralized plating solution.
This method, however, has a disadvantage of causing errors that make analytical values smaller than correct ones. This is inevitable because the plating solution is adjusted to pH 9-10 before the addition of sodium sulfite in order to avoid the effect of other components in the plating bath. At this pH level, the above-mentioned reaction is slow and incomplete, particularly where the concentration of formaldehyde is low. In practice, the analytical values obtained by this method are corrected by an empirical factor. Moreover, this method has another disadvantage attributable to the use of a pH meter. The glass electrode of a pH meter tends to cause errors when deteriorated by the strong basicity (pH 13-14) resulting from sodium sulfite added.
Recently, a new type of electroless copper plating bath has appeared. It is incorporated with a compound such as glycine which forms an addition product with formaldehyde, so that the activity of formaldehyde is controlled. In using this plating bath, it is necessary to control the concentration of free formaldehyde which remains unreacted with the compound to form an addition product. Free formaldehyde greatly affects the deposition rate of the electroless copper plating and the physical properties of the copper deposits. The sodium sulfite method has a disadvantage that it determines total formaldehyde, but does not selectively determine free formaldehyde.
In addition to the above-mentioned method, there is known a titrimetric method that employs an iodine solution for the determination of formaldehyde in the plating bath. This method is limited in application and impracticable, because iodine can react with a complexing agent or other components in the plating bath.
In consideration of the foregoing, the present inventors carried out extensive studies on the analytical method for determining formaldehyde in the electroless copper plating bath, with great emphasis on the method that gives accurate results even in the case where the concentration of formaldehyde is low and on the method capable of determining free formaldehyde directly in the case where the plating solution is incorporated with a compound that forms an addition product with formaldehyde. As the result, it was found that this object can be achieved by potentiometric titration in which a hydroxylamine salt such as hydroxylamine hydrochloride is used as the titrant and a silver electrode as the indicator electrode. The present invention was completed based on this finding.